Vinod K. Jain

An evaluation of parameters for predicting fretting fatigue crack initiation

Abstract There are numerous fatigue parameters that can be used to determine the onset of crack initiation in a component subjected to constant amplitude plain fatigue. This study evaluated how well some of these parameters predict fretting fatigue crack initiation in titanium alloy, Ti–6Al–4V. The following crack initiation parameters were evaluated; the strain-life parameter, the maximum strain corrected for strain ratio effects, the maximum principal strain corrected for principal strain ratio effects, the Smith–Watson–Topper (SWT) parameter, the critical plane SWT parameter and the Fatemi and Socie (F–S) parameter. The Ruiz parameters, which are specific to fretting fatigue condition, were also evaluated. The evaluation was based on the parameters ability to predict the number of cycles to initiation and location for crack initiation. The results indicated that the maximum strain amplitude at the contact interface was an important parameter for fretting fatigue crack initiation. Furthermore, the results indicated that when the applied loading was corrected for the effects of contact and mean stress or strain ratio, titanium alloy, Ti–6Al–4V exposed to the fretting fatigue condition behaved in a manner similar to the plain fatigue condition.

Combined experimental–numerical investigation of fretting fatigue crack initiation

Abstract This study investigated the fretting fatigue crack initiation behavior of titanium alloy, Ti–6Al–4V. Tests were conducted to generate fretting fatigue failures from 2×104 to 5×107 cycles at 200 Hz. Fractography was employed to determine number of cycles to crack initiation, crack location and angle of crack orientation. Finite element analysis was conducted based on the experimental information in order to assess the ability of two critical plane approaches to predict fretting fatigue crack initiation behavior; the Smith–Watson–Topper critical plane parameter and the maximum shear stress range critical plane parameter. When properly formulated, these parameters predicted number of cycles to crack initiation and location of crack initiation which were in agreement with the experimental counterparts. However, these two parameters predicted different orientation angles of crack initiation at the contact surface. Based on the observations of orientation angles, the combined experimental–numerical approach showed that the mechanism for fretting fatigue crack initiation was governed by the maximum shear stress range on the critical plane.

The effect of diamond-like carbon coatings on the rolling fatigue and wear of M50 steel

Abstract The rolling contact fatigue and wear characteristics of uncoated and diamond-like carbon coated VIM-VAR M50 bearing steel were investigated at room temperature and 177°C (350°F) and a Hertzian stress level of 4.8 GPa (700 ksi). The coatings were deposited via ion beam enhanced deposition and were approximately 33 nm thick. Rolling fatigue and wear tests were conducted using a ball-on-rod type tester. Results did not indicate any significant difference in the fatigue life of coated and uncoated specimens at room temperature, at 90% confidence level. However, the coating significantly improved the fatigue life (90% confidence level) at 177°C and wear resistance at both temperatures. Some correlation was noted between wear and fatigue life for the uncoated specimens at both temperatures, but none for the diamond-like carbon coated (the fatigue life was independent of wear) specimens. Scanning electron microscopy did not show any sign of coating delamination.

Microstructure Development during Conventional and Isothermal Hot Forging of a Near-Gamma Titanium Aluminide

The breakdown of the lamellar preform microstructure in the ingot metallurgy near-gamma titanium aluminide, Ti-45.5Al-2Cr-2Nb (atomic percent), was investigated. Microstructures developed during canned, conventional hot forging were compared to those from isothermal hot forging. The higher rate of deformation in conventional forging led to considerably finer and almost completely broken-down structures in the as-forged condition. Several nontraditional approaches, including the isothermal forging of a metastable microstructure (so-called “alpha forging”) and the inclusion of a short static recrystallization anneal during forging, were found to produce a more fully broken-down structure in as-isothermally forged conditions. Despite the noticeable microstructure differences after forging, conventionally and isothermally forged material responded similarly during heat treatment. In both cases, almost totally recrystallized structures of either equiaxed gamma or transformed alpha grains surrounded by fine gamma grains were produced depending on the heat-treatment temperature. Metallography on forged and heat-treated pancake macroslices was useful in delineating small differences in composition not easily detected by analytical methods.

Determination of heat transfer coefficient for forging applications

A setup for measuring the contact heat transfer coefficient between a tool steel die and a specimen was designed, developed, and fabricated. A microcomputer was programmed to collect and process the temperature data. Tests were conducted under dry and lubricated conditions at different temperatures and pressures on four aluminum alloys, namely, 2024-T4, 2024-O, 6061-O, and 1100-O. MoS2 was used as the lubricant. Results indicate that the heat transfer coefficient increases with pressure and decreases with specimen yield strength. Variation in heat transfer coefficient with temperature is due to chemical changes which occur in the lubricant at the test temperature. The heat transfer coefficient was an order of magnitude greater at high pressures than at nominally zero pressure.

Investigation of the wear mechanism of carbon-fiberreinforced acetal

Abstract The friction and wear characteristics of acetal reinforced with 25 wt.% of randomly dispersed carbon fibers were investigated and compared with those of the base resin. The addition of carbon fibers to the polymer decreased the coefficient of friction and wear rate for the composite material sliding against a hardened and ground steel disk for extended periods of time. A number of surface topography parameters pertinent to the wear process, i.e. arithmetic average roughness, autocorrelation function, asperity density, tip radius of curvature, distribution of the radii of curvature and heights of asperity peaks, and ordinale heights, were calculated and evaluated for the sliding surface of the disk. The worn surfaces of filled and unfilled polymer pins were examined by scanning electron microscopy to investigate the probable wear mechanism for these materials. It was found that the wear of reinforced polymers proceeds by the pulling-out of the broken and worn fibers.

Computer Simulation of the Forging of Fine Grain IN-718 Alloy

In recent years, there has been great emphasis on the use of computer-aided tools in process design. The key to the success of any computer modeling is the accurate knowledge of the mechanical and thermal properties of the various components of a manufacturing system. In order to develop a data base of forging properties of the nickel-base alloy IN-718, isothermal constant strain-rate compression tests were conducted on the annealed fine-grain material over the temperature range 871 °C to 1149 °C (1600 °F to 2100 °F) and strain-rate range 0. 001 to 10 s−1. Empirical relationships among flow stress, strain rate, and temperature developed based on these tests, along with experimentally measured heat-transfer and friction coefficients, were used in the program ALPID to simulate nonisothermal forging of “double-cone” specimens. The simulation results were compared with actual forging in an industrial forge press. The good agreement between simulation and forging results indicates that when a complete data base of materials properties is available, computer modeling can be used effectively to study the forging process.

Processing of an experimental aluminum–lithium alloy for controlled microstructure

Abstract This paper describes the processing route that was used to obtain unrecrystallized and recrystallized microstructural versions of an experimental Al–Li alloy. The alloy, designated AF/C-489, has a nominal chemical composition of Al–2.05 Li–2.7 Cu–0.3 Mg–0.6 Zn–0.3 Mn–0.04 Zr (weight percentage). Processing schemes to obtain unrecrystallized and recrystallized conditions of the Al–Li plate were obtained by performing dynamic material modeling. Dynamic material modeling involved compression testing over strain rates ranging from 0.05 to 10 s −1 and temperatures ranging from 300 to 550°C and generating a processing map. Rolling parameters were obtained by using the constitutive equations obtained from dynamic material modeling as input to the finite-element analysis code.

Physical modeling of metalworking processes—I: Determination of large plastic strains

This article discusses the application of the visioplasticity method to the evaluation of large plastic strains such as those occurring in metalforming. Although this method can be used for any mode of deformation, its application to plane-strain deformation is treated here. The distortion of a quadrilateral element of a grid is tracked to compute strains during deformation. In each case any two lines of the quadrilateral can be used (length before and after deformation and direction cosines before deformation) to determine strains in the element. The method has been verified by application to basic cases of deformation such as uniaxial compression, tension, pure shear, and rotation of elements. The effect of choice of lines upon the results of strain calculation is also discussed.

Effect of core insulation on the quality of the extrudate in canned extrusions of γ-titanium aluminide

Abstract Non-isothermal forward extrusions of cast γ-TiAl (Ti-49.5Al-2.5Nb-1.1Mn, at%) were performed with billet temperatures of 1050, 1150, and 1225°C and a die and container temperature of 260°C. The billets were canned using 304 stainless steel tubing. Finite-element analysis indicates that rapid heat loss occurs from the canned billet to the die during the extrusion process. The temperature gradient in the billet produces non-uniform microstructure. A technique was developed to insulate the billet with layers of 0.050 inch (1.27 mm) thick silica fabric. The use of an insulator reduces billet heat loss during its transfer from furnace to press and during the extrusion process. Differences in microstructure and mechanical properties of the extrusions performed with and without the silica insulation were investigated and compared.